Production of paraxylene



April 6, 1965 N. N. BUCHSBAUM ETAL PRODUCTION OF PARAXYLENE FilBd March 27, 1961 FRESH FEED v7 I I PREHEAT'ER. 148

- CRISI'ALLIZER.

42 scmnxron PARAXYLENE ISOMERIZER.

LIGHT 2/ MATERIAL serum-on.

FLASH POT FRACT. PM

HEAVY MATERIAL INVENTORS NORBERT M BUCHSBAUM MART/N Q SORENSEN ATT EY United States Patent ()fiice 3,177,254 Patented Apr. 6, 1965 3,177,264 PRODUCTION OF PARAXYLENE Norbert N. Buchshaum, Passaic, N .L, and Martin 0.

Sorensen, New York, N. Y., assignors to Halcon International, Inc., a corporation of Delaware Filed Mar. 27, 1961, Ser. No. 98,581

2 Claims. (Cl. 260-674) This invention is concerned with the preparation of isomeric aromatic hydrocarbons; more particularly, it is concerned With the preparation and separation of C-8 aromatic hydrocarbons. In a particular embodiment, the invention is'concerned with the production of paraxylene by the conjoint use of crystallization and isomerization techniques.

In a particular embodiment the present invention is concerned with treatment of the effiuent of a C-8 isomerizer in order to separate the'isomerizate from light and heavy ends.

Paraxylene is a chemical of growing commercial importance, being utilized, for example, as a precursor for terephthalic acid, one of the constituents of polyester fibers and resins. In the past, paraxylene has been separated by crystallization from various C-8 aromatic hydrocarbon refinery streams. More recently, paraxylene has been obtained by such crystallization technique followed by isomerization of the crystallizer effluent to enrich the paraxylene content thereof. Such process is illustrated by Petroleum Refiner, August 1958, page 208.

In one embodiment the present invention embraces in sequential operation for the production of paraxylene, the steps of separating by crystallization paraxylene from a mixture consisting predominantly of aromatic hydrocarbons, said mixture containing paraxylene and at least one other xylene isomer, subjecting the residuum of said aromatic hydrocarbon mixture to isomerization conditions to produce a mixture of paraxylene-enriched isomerizate, a lighter than C8 fraction and a heavier than C-8 fraction, introducing said mixture into a first fractionation zone, removing heavier than C-8 fraction from the bottom of a first fractionation zone, removing said lighter than C-8 fraction and said isomerizate as overhead from said first fractionation zone, introducing said lighter than C-8 fraction and said isomerizate into a second fractionation zone, removing said lighter than C-8 fraction as overhead from said second fractionation zone and said isomerizate from the bottom of said second fractionation zone, admixing said isomerizate and fresh aromatic hydrocarbon feed to form a mixture consisting of predominantly aromatic hydrocarbons and repeating the sequential operation. I

In a further embodiment the present invention embraces in a sequential operation for theproduction of paraxylene, the steps of separating by crystallization paraxylene from a mixture consisting predominantly of aromatic hydrocarbons, said mixture containing paraxylene and at least one other xylene isomer, subjecting the residuum of said aromatic hydrocarbon mixture to isomerization conditions to produce a second mixture of a paraxylene-enriched isomerizate, a lighter than C-8 fraction and a heavier than C-8 fraction, partially flashing said second mixture and introducing said partially flashedmixture into a first fractionation zone, removing the heavier than C8 fraction from'the bottom of said first fractionation Zone, removing said lighter than C-8 fraction and said isomerizate as overhead from said first fractionation zone, introducing said lighter than C-8 fraction and said isomerizate into a second fractionation zone, removing said lighter than (1-8 fraction as overhead from said second fractionation zone and said isomerizat'e from the bottom of said second fractionation zone, admixing said isomerizate and fresh aromatic hydrocarbon feed to form a mixture consisting predominantly of aromatic hydrocarbons, and repeating the sequential operation. 7

The present invention is illustrated by the attached drawing in which a 0-8 aromatic hydrocarbon stream such as is obtained from a refinery stream is introduced through line 11 together with a recycle isomerizate (hereinafter described) from line l2 into crystallization zone 13. Crystalline paraxylene is removed from said zone 13 via line 14 by means such as centrifugation. The residuum of the aromatic hydrocarbon stream together with hydrogen from line 16 is fed via line 15 to the preheater 17 and thence via line 18 to isomerizer 19. The crude isomerizate efiluent flows via line 211 through heat exchanger 22 to a liquid-gas separator 23. The hydrogen gas is recycled via line 24 with make-up hydrogen being introduced via line 25. The crude isomerizate effluent, containing a lighter than C-8 fraction, a paraxylene enriched isomerizate and a heavier than C-8 fraction, flows via line 26 to flash pot 27 where a portion of the crude isomerization effluent is flashed, and the liquidvapor is transported via line 28 to a first fractionation Zone 31. The heavy ends of said crude isomerization efiiuent are removed from first fractionation zone 31 as bottoms via line 32. An overhead vapor stream containing the isomeric C-8s and the lighter than C-8s is removed from first fractionation zone 31 via line 33 and fed to second fractionation zone 34-. The lighter than C-8s are removed as overhead from second fractionation zone 34 via line 35. The recycle isomerizate is removed from fractionation zone 34 via line 12 and combined with fresh feed.

The procedure of the present invention enables more economic operation than heretofore known in the art.

For example, in previous methods of operation, the

efiluent from the isomerizer was partially vaporized as 7 above described and introduced into a first fractionation zone from which the heavy ends and isomerizate were removed as bottoms and the light aromatics as overhead. This necessitated the vaporization of the bottoms from the first fractionation zone for introduction into a second fractionation zone from which the recycled isomerizate was removed as overhead and the heavy ends as bottoms. Thus, the present invention provides a distinct advantage in that, utilizing the present invention, greatly reduced vaporization heat requirements result as compared to prior processes, and economic advantage obviously accrues- The invention is illustrated by but not necessarily re stricted to the following examples in which parts by Weight are percent by Weight apply unless otherwise indicated.

i Example 1 About parts of fresh feed having a composition of about 23% ethylbenzene, 17% paraxylene, 37%

sava es metaxylene; 23% othoxylene and the rest hydrocarbon impurities are combined with about 500 parts recycle isomerizate having a composition of about 17% ethylbenzene, 17% paraxylene, 38% metaxylene, 26% orthoxylene and the rest lower and higher boiling impurities. The resulting mixture is fed to a crystallization and separation zone wherein paraxylene' crystals are formed by cooling to about 70 C. and are separated by centrifugation. 'Approxirnately 50-60 parts of paraxylene crystals are removed.

The mother liquor is combined with hydrogen (about 450 parts) preheated and 'fiowed through isomerization zoneat about 460 (Land 190 p.s.i.g. whereby the para-- 1 xylene depleted mixture isisomerized and there is obtained a crude isomerizer effluent containing a paraxylene enriched isomerizate a heavier than C-S fraction and a lighter than C-8 fraction. The crude isomerizer effluent together with the excess hydrogen is passed through I a liquid gas separator wherein hydrogen is disengaged and recycled. The crude isomerizer efiluent comprising about 15% ethylbenzene, 16% paraxylene, 35% metaxylene, 24% orthoxylene, 7% lighter than C-Ss and 3% heavier than C-8s is partially flashed at a temperature of about 40 C. and. 170'p.s.i.g. and the vapor-liquid mixture is fed into the first fractionation zone'where there are removed as bottoms at 203 C. and 31 p.s.i.g. approximately 10-15 parts of a heavier than -8 fraction. The lighter than (3-8 fraction and the paraxylene enrichedisomerizate are removed from the first 'fractionation zone as overhead at 175C. and 25 p.s.i.g. and fed to a second fractionation zone wherein approximately 35-45 parts of a lighter than C-8 fraction are removed at 118 C. and 16 p.s.i.g. as overhead, and approximately 7 500 parts of the paraxylene enriched isomerizate are obtained as bottoms at 171 C; and 20 p.s.i.g. and recycled for combination With the fresh feed.

Utilizing this sequence, there is a total heat duty, in the first and second fractionation zone of about 360 parts of steam per 100 parts of crude isomerizer effluentfed,

Example 2 The procedure of Example 1 is repeated except that the 35-45 parts of lighter than C-8 fraction is removed as overhead into the first fractionation zone and from the bottom of said fractionation zone there is removed the heavier than C-8 fraction and the paraxylene'enriched isomerizate which are fed to the second fractionation zone. In the second fractionation zone the approximately 10-15 parts of heavier than (3-8 fraction are removed as bottoms and approximately 500 parts paraxylene enriched isomerizate removed as overhead for recycle and combination with the fresh feed.

Utilizing this method of operation heat duty in the first and second fractionation zone is about 550 parts steam per 100 parts crude isomerizer efiiuent fed;

V the range:

The crystallization is generally'carried out in the range +5 to74 C., and, any-suitable heat exchange means may be used such as internal or direct contact with a- Ethylbenzene 0-30 OrthoXylene I -25 Paraxylene 16-19 Metaxylene 25-50 Heavier than C-8 2-5 Lighter than C-8 2-10 refrigerant such as ethylene, or indirect contact with the refrigerant. If desired, scraper means may be provided for removing or loosening crystals or solids from heat exchange surfaces. One or more stages of cooling and separating may be used. V

The solids separator may be any convenient means, such as a filter, centrifuge, or centrifugal basket. I

The isomerization is generally carried out at 425 C. to 500- C. at a pressure of 150-255 p.s.i.g., preferably using 0 to 10 volumes of hydrogen per volume of hydrocarbon vapor. V The is'omerizate efiiuent afterseparation of the'hydrogen has a composition generally in the range:

7 Percent The refined isomerizate has a composition generally in the range:

7 Percent Ethylbenzene a 0-30 Orthoxylene 15-25 Paraxylene 16-20 Metaxylene 25-50 Other hydrocarbonsup to 1.5.

The second fractionator overhead is usually about 145 C. and-2 p.s.i.g.to- 25 p.s.i.g., and the'bottoms 110-180 C. and 7 p.s.i.a. to 30 p.s.i.g.

What is claimed is: V i a Y 1. In a process for the production of paraxylene wherein the paraxylene is crystallized and separated from a mixture containing predominantly C aromatiohyd'rocarbons; and wherein the residual liquid is isomerized to form an isomerizate enriched in paraxylene, the improvement of: initially fractionating said'isomerizate in'a first fractionation zone to remove materials heavier than. C aromatic hydrocarbons asa bottoms fraction; subsequently introducing the distillate from said first fractionation zone; as

' avapor to a second fractionation zone; fractionating said distillate in said fractionating zone to remove materials lighter than C aromatic hydrocarbons as a distillate and recycling the bottoms product from said second fractionation zone containing a'purified-C aromatic hydrocarbon to said'crystallization step.

2 The process of claim 1 wherein the first fractiona- I tion zone is maintained at an overhead temperature of from to 180 C; and pressure of 2 p.s.i.a, to 40 p.s.i.g.,

' at a bottoms temperature of 150 to 210 C. and pressure Percent Ethylbenzene 0-30 Orthoxylene 15-35 Paraxylene 12-25 Metaxylene 25-50 Other hydrocarbons. -e trace-2 of 12 to 50 p.s.i.g.; and said second fractionation zone at an overhead temperature of about 45 to C. and

7 pressure of 2 p.s.i.g. to-25 p.s.i.g., and a bottoms temperature of 110 to C., and pressure from 7 p.s.i.a. to30 p.s.i.g. I r

, References Cited by: the Examiner UNITED; STATES PATENTS 2,837,581 6/58 Hill etal; -2s0 674 

1. IN A PROCESS FOR THE PRODUCTION OF PARAXYLENE WHEREIN THE PARAXYLENE IS CRYSTALLIZED AND SEPARATED FROM A MIXTURE CONTAINING PRREDOMINANTLY C8 AROMATIC HYDROCARBONS; AND WHEREIN THE RESIDUAL LIQUID IS ISOMERIZED TO FORM AN ISOMERIZATE ENRICHED IN PARAXYLENE, THE IMPROVEMENT OF: INITIALLY FRACTIONATING SAID ISOMERIZATE IN A FIRST FRACTIONATION ZONE TO REMOVE MATERIALS HEAVIER THAN C8 AROMATIC HYDROCARBONS AS A BOTTOMS FRACTION; SUBSEQUENTLY INTRODUCING THE DISTILLATE FROM SAID FIRST FRACTIONATION ZONE; AS 